The present invention relates to a battery and particularly to a battery to be incorporated in small-sized electronic equipments.
In a battery having a metallic can as a container, it has been heretofore practiced to press the electrodes under a predetermined pressure and make the distance between the electrodes even. This is because that, when uniform distance between the electrodes along the surface of the electrodes is usual in that the electrode reaction proceeds uniformly all over the electrodes, expecting a prolonged life.
In recent years, a thin battery using a container of, e.g., a metal-resin laminate film rather than metallic can has appeared. This battery comprises an electricity-storing element made of a positive electrode, a separator and a negative electrode received in an airtight bag obtained by adhering a laminate film at the edges thereof.
However, since this type of a battery comprises, as a battery container, a container made of a flexible laminate film rather than a metallic can, the electrodes cannot be pressed by the pressure of the battery container. Thus, the distance between the electrodes is ununiform, causing a remarkable drop in the capacity during charge and discharge cycles.
Thus, Japanese Patent Application Laid-Open No. 1998-302843 proposes that the separator and the electrodes be bonded to each other with an adhesive. In accordance with this proposal, the distance between the electrodes can be kept constant even without any pressure of the battery container, causing the electrode reaction to proceed uniformly all over the electrodes and hence giving a prolonged life. As the adhesive used, ethylene glycol dimethacrylate, methyl methacrylate or the like is dissolved in N-methylpyrrolidone or the like.
However, when such an adhesive is used, a dense adhesive layer is formed on the surface of the electrodes. Accordingly, this layer of adhesive was disadvantageous in that it prevents the movement of the electrolyte across the space between the electrodes, causing an energy density drop.
It is therefore an object of the present invention to provide a battery which exhibits a high energy density and an excellent cycle life performance even if the electricity-storing element is received in a flexible material case.
The battery of the present invention is a battery comprising a positive electrode, a negative electrode and a separator provided interposed therebetween, wherein at least one surface of said separator is bonded to said positive electrode or negative electrode via a porous resin layer comprising a solid filler.
In the battery of the invention, since the electrode and the separator are bonded to each other with a porous resin layer as mentioned above, the distance between the electrodes can be kept constant even if the battery container is so flexible that the pressure of the battery container is not sufficient. Accordingly, even when subjected to repeated charges and discharges, the battery of the invention shows no capacity drop and thus exhibits a prolonged life.
Further, since the resin layer is made porous by the addition of a solid filler, the electrolyte can move across the space between the electrodes through the pores formed in the resin layer, enhancing the energy density.
The thickness of the resin layer is preferably from 1 xcexcm to 10 xcexcm. This is because the energy density of the battery can be enhanced when the thickness of the resin layer falls within this range. In other words, when the thickness of the resin layer falls below 1 xcexcm, the adhesion between the electrode and the separator becomes insufficient. Therefore, it is likely that when the electricity-storing element is received in the battery container or the battery is in use, the distance between the electrodes can become ununiform, causing a capacity drop with the repetition of charge and discharge. On the contrary, when the thickness of the resin layer exceeds 10 xcexcm, the distance between the electrodes becomes too long, causing an energy density drop.
The thickness of the separator is preferably not greater than 25 xcexcm. This is because the energy density of the battery can be enhanced when the thickness of the separator falls within this range. In other words, when the thickness of the separator exceeds 25 xcexcm, the distance between the electrodes becomes too large, causing an energy density drop.
The resin to be used in the resin layer is not specifically limited but preferably comprises at least one member selected from the group consisting of polyethylene, polypropylene, poly(vinylidene chloride), poly(vinylidene fluoride), poly(ethylene oxide) and polyacrylonitrile.
Alternatively, the resin to be used in the resin layer preferably comprises at least one member selected from the group consisting of copolymer of vinylidene fluoride and hexafluoropropylene, copolymer of vinylidene fluoride and chlorotrifluoroethylene, copolymer of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene, copolymer of vinylidene fluoride and tetrafluoroethylene and copolymer of hexafluoropropylene and tetrafluoroethylene.
The solid filler to be used in the resin layer preferably comprises a ceramic powder made of primary particles having an average diameter of from 5 to 100 nm.
This is because, when a resin solution containing a ceramic powder falling within this range is dried, the resin solution is adsorbed to the ceramic powder during drying. A lower amount of the resin solution exists less in the portion other than the portion having the ceramic powder present therein and thus forms pores when dried, rendering the resin layer porous. When the particle diameter of the ceramic powder exceeds 100 xcexcm, the resin is adsorbed less to the ceramic powder, making it impossible to make the resin layer uniformly porous and hence causing a capacity drop and a resistance rise.
The ceramic powder is not specifically limited but preferably comprises at least one member selected from the group consisting of alumina, silica, titania and zirconia. This is because these materials are all excellent in resistance to organic electrolyte.
The specific surface area of the solid filler is preferably from not smaller than 50 m2/g to not greater than 500 m2/g. This is because when the specific surface area of the solid filler falls below 50 m2/g, the resin is adsorbed less to the ceramic powder, making it impossible to make the resin layer uniformly porous and hence causing a capacity drop and increase of the resistance. This is also because when the specific surface area of the solid filler exceeds 500 m2/g, the amount of a solvent to be adsorbed to the ceramic powder increases during the preparation of a paste of the resin, the ceramic powder and the solvent, making it difficult to form a uniform resin layer and hence lowering the adhesion strength, which deteriorates the cycle life performance.
A part of the resin layer preferably penetrates into the surface layer of the positive electrode and negative electrode. This is because, when a part of the resin layer penetrates into the surface layer of the positive electrode and negative electrode, the separator can be firmly bonded to the positive electrode and negative electrode to keep the distance between the electrodes constant. Thus, when subjected to repeated charges and discharges, the battery undergoes no capacity drop and hence exhibits a prolonged life.
The battery of the invention may be applied to any type such as cylindrical battery, prismatic battery, sheet-shaped battery, laminated battery, coin-shaped battery and pin-shaped battery. The shape of the battery of the invention is not specifically limited, but the positive electrode, the negative electrode and the separator are preferably received in a flexible material case. The distance between the electrodes can be difficultly kept constant particularly when the battery container is flexible. Even in such a case, the present invention makes it possible to keep the distance between the electrodes constant, causing no capacity drop and hence giving a prolonged life.
The battery of the present invention can be widely used regardless of which it is of primary type or secondary type.